powerpc/time: Make get_tbl() common to PPC32 and PPC64

On PPC64, get_tbl() is defined as an alias of get_tb() which return
the result of mftb(). That exactly the same as what the PPC32 version
does. We don't need two versions.

Remove the PPC64 definition of get_tbl() and use the PPC32 version
for both.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a8eaabb87d69534e533ebac805163e08146e05bd.1601556145.git.christophe.leroy@csgroup.eu